The specific way Scala is bound to the JVM (as a 8mb class library that needs to be loaded every time) is the reason it is slow to compile and by implication, slow-er startup.
I suppose we do need to take account of language features. C++ is a difficult language to parse, and with a bit of template magic, and heavy compiler optimizations that can be applied. Haskell has type inference and is statically typed, that has to cost it some what.
Scala has a 8mb JVM library to load, and also type inference. It gets a double whammy. However I'm willing to bet is the JVM is contributing a significant portion of startup time, with both JVM startup time, loading java libraries, then loading scala library. If the JVM wasn't part of the problem, then there is no need for people to create tools like fsc to speed up compile time.
